Tapping into vacuum line Question

[predator]

T it from the std. map sensor that gives the stock boost guage its reading.

:werd: best place for it.. its little tube that runs into the back of your plenum..

[Sydneykid]

and thats the little black box on the firewall on the drivers side???

YES:cheers:

[druzilla32]

sick fully

got it done.. wasnt to hard getting the pipe thru the firewall.. still, it had me in tears!

[Sydneykid]

Nissan designed the vacuum hose for the fuel pressure regulator supply to be nice and short for very good reasons. That is response, the short hose means the fuel pressure regulator responds to changes in the plenum pressure (vacuum or boost) very quickly. Plus reliability, the hose is nice and short therefore less chance of it cracking or being broken.

Firstly, by sticking a long vacuum hose (to the boost gauge in the cabin) you are introducing a dampening effect of the boost/vacuum changes on the fuel pressure regulator. This means it responds slower to changes. Not a good idea.

Secondly, you are introducing a huge increase in potential for failure. Loosing the boost supply to the fuel pressure regulator can be catastrophic. Let's do some numbers;

The fuel pressure regulator is set for 38 psi above boost (that's ~standard for an RB)

You are running 20 psi of boost.

That means 58 psi in the fuel rail.

The hose to the boost gauge splits

You loose 20 psi of fuel pressure

The fuel rail pressure drops to 38 psi less the 20 psi of boost = (effectively) 18 psi at the injector outlet

The fuel flow drops (38 psi versus 18 psi) by ~35%

What was a nice safe 11.5 to 1 air fuel ratio drops to 15.5 to 1

You now have 6 pistons with holes in them after less than 10 seconds of full power running

Personally I see no good reason to take that sort of risk.

This is not the same as the vacuum supply to the boost gauge splitting when it is connected to the plenum (not in line with the fuel pressure regulator). It that happens then some boost will escape, not much as its small hose. But the injectors will continue to squirt the same amount of fuel. This means you will have slightly rich running, which as everyone knows is much, much safer than huge lean running.

As for Blitz/HKS/Apex etc not telling you the wrong thing, give me a break. They have only one diagram for all engines! And its been the same one for 10 years. If they told you the best plumbing for all engines they would have to have hundreds of drawings and they would have to be updated every time a new engine/vacuum system was introduced. They are simply not going to do that, so they rely on the simplest and most common. Every turbo car has a vacuum line to the fuel pressure regulator, plumb it in there it’s the same for all cars. One drawing will do then, thank you very much.

But we on SAU know better than that and we can share that detailed knowledge on Skylines with you. Take advantage of it or not, I don't really care, it's your engine after all.

[someonestolecc]

Thanks for the post SK I will check my lines thanks to your post.. My car was put together dodgily so I'd like to have piece of mind as as you quite succintly put it is (now) my engine.

[Godzil_R]

cheers sydneykid, Good explanation(make sense).

[someonestolecc]

Sorry to bring an old thread up but I've had a look into this and it appears my boost gauge has been tapped into a hose between the plenum and fuel rail.

This is in a SR20 so it's a bit different to the RB20.

I'm wondering if anyone knows the location of the best place to tap it into the SR20 - I've had a look around for what's been described but unfortunately I don't know where to find it.

With the RB20 and from the conversations and advice in this thread the place seems to be at the back of the engine near the firewall, just behind the intake where the intercooler pipe goes in. Does that sound right? (Want to make sure before we do my mates car).

[siksII]

my car was idling funny and did not show as much vaccum as normal the last time or two i have driven it, i had a quick look tonight and found out that the vaccum hose going to the FPR from the plenum had come off the FPR.

there has been no unusual knocking shown on the powerfc, so i am assuming it must not have leaned out too much? it has not knocked at all and i have floored it plenty of times, i am running 10psi on rb25...

could there be other damage?